Field of the Disclosure
The present application relates to lasers suitable for generating radiation at deep UV (DUV) and vacuum UV (VUV) wavelengths, and to methods for generating laser light at DUV and VUV wavelengths. In particular, the present application relates to systems and methods for reducing and controlling the spectral bandwidth of DUV and VUV lasers. The lasers described herein are particularly suitable for use in inspection systems including those used to inspect photomasks, reticles, and semiconductor wafers.
Related Art
The integrated circuit industry requires inspection tools with increasingly higher sensitivity to detect ever smaller defects and particles whose sizes may be 100 nm or smaller. Furthermore, these inspection tools must operate at high speed to inspect a large fraction or even 100% of the area of a photomask, reticle or wafer in a short period of time, e.g. one hour or less.
Generally, short wavelengths such as DUV and VUV wavelengths have higher sensitivity for detecting small defects compared with longer wavelengths. Inspection of a photomask or a reticle is preferably done using the same wavelength as the lithography used when printing from the photomask or reticle. Currently, a wavelength of substantially 193.4 nm is used for the most critical lithography steps and a wavelength of substantially 248 nm is used for less critical lithography steps.
High-speed inspection requires high power lasers to illuminate the samples being inspected with high intensity to detect the small amount of light scattered from small particles or defects or allow detection of small changes in reflectivity due to defects in the pattern. The required laser power levels may range from approximately 100 mW for the inspection of photomasks and reticles up to more than 10 W for the detection of small particles and imperfections on a bare silicon wafer.
Typically, inspection in the semiconductor industry requires lasers with very narrow bandwidth. Such inspection systems usually use an objective lens with a large field of view (typically from a few hundred microns to a few mm in dimensions) to allow imaging of a large area at high inspection speeds. An objective lens with low distortions and a large field of view is expensive and complex. Requiring that objective lens to operate over a large bandwidth (such as more than a few tens of pm) significantly increases the cost and complexity. DUV lasers with bandwidths of approximately 20 pm or less are very desirable for inspection applications in the semiconductor industry.
DUV lasers are known in the art. U.S. Pat. No. 5,144,630 entitled “Multiwave Solid State Laser Using Frequency Conversion Techniques” that issued on Sep. 1, 1992 to Lin, and U.S. Pat. No. 5,742,626, entitled “Ultraviolet Solid State Laser Method Of Using Same And Laser Surgery Apparatus”, issued on Apr. 21, 1998 to Mead et al. describe exemplary DUV lasers. In these lasers, fourth and fifth harmonics are generated from a pulsed fundamental infra-red laser operating at a wavelength near 1064 nm, thereby resulting in wavelengths of approximately 266 nm and 213 nm. Lin and Mead also teach generating an infra-red wavelength longer than 1064 nm from the fundamental laser using an optical parametric oscillator (OPO).
The output bandwidth of a laser oscillator is determined by its intra-cavity dynamics. In prior art pulsed lasers, to further reduce laser bandwidth, various bandwidth limiting devices, such as an etalon, a birefringent filter, or an optical grating, have been incorporated into a laser cavity. Because all of these approaches are invasive, they inevitably introduce detrimental effects to the lasers. These detrimental effects include extra power losses and greater complexity, which often lead to lower laser efficiency, poor thermal stability, tighter misalignment sensitivity, and longer laser system warm-up time. Furthermore, because intra-cavity beam size is often small and predetermined by laser cavity design, and intra-cavity laser power density is normally much higher than laser output power, these intra-cavity components are much more susceptible to damage.
In prior art pulsed DUV lasers, the bandwidth of the DUV output depends directly on the bandwidth of the fundamental infra-red laser. That is, the broader the bandwidth of the fundamental laser, the broader the DUV output bandwidth. Reducing the bandwidth of a laser requires redesigning the laser oscillator cavity. Because the cavity may control many properties of the laser including bandwidth, repetition rate, as well as average and peak powers, redesigning the cavity to reduce the bandwidth while maintaining the other laser parameters may be a complex and time consuming task. Furthermore, achieving a specific DUV laser bandwidth specification may not be possible using a readily available infra-red fundamental laser.
It is well-known that a chirp stretches the length of a laser pulse and reduces its peak power (see, for example, http://www.rp-photonics.com/chirp.html). As non-linear conversion efficiency scales with peak power, a lower peak power would reduce the overall conversion efficiency, thereby limiting the maximum UV power generated from a laser system. Therefore, for a given required bandwidth, close to transform-limited (also called “chirp-free”) pulses are desirable for high non-linear conversion efficiency. However, because of laser intra-cavity dynamics, such as dispersion, spatial-hole burning (SHB), gain saturation, and non-linearity, pulses generated from lasers are often chirped.
Therefore, a need arises for DUV laser overcoming some, or all, of the above disadvantages. In particular, a need arises for a means of reducing or controlling the bandwidth of a DUV laser.